1. Field of the Invention
The present invention relates to a method of determining the value of a driving voltage applied to a piezoelectric ink jet print head and adjusting the ink ejection condition of the print head.
2. Description of Related Art
An ink jet print head is provided in an ink jet printer. There has been proposed that a piezoelectric type ink-jet print head unit is employed as the ink jet print head. In this type of print head unit, a piezoelectric element is provided to each pressure chamber. By applying an electric voltage to the piezoelectric element, the volume of the corresponding pressure chamber decreases, thereby causing ink to be ejected through a nozzle from the pressure chamber.
In order to manufacture this type of print head unit, a plurality of piezoelectric ceramic sheets are prepared. Internal electrode layers are screen-printed on the piezoelectric ceramic sheets. The piezoelectric ceramic sheets are stacked one on another, pressed against one another, and then baked into a single actuator. In the thus produced actuator, a plurality of piezoelectric elements are defined by a plurality of internal electrode sections, which are defined by the internal electrode layers.
A cavity plate is formed with a plurality of pressure chambers. The actuator is then bonded to the cavity plate so that the piezoelectric elements are located in one to one correspondence with the pressure chambers.
A nozzle plate is made from polyimide or the like, and is formed with a plurality of nozzles. The nozzle plate is bonded to the cavity plate, thereby finally obtaining the ink jet print head unit.
There are, however, variations in the diameters of nozzles formed in the nozzle plate. There are variations also in the characteristics of the piezoelectric elements. Due to these variations, there are variations in the ink ejection performances in the individual ink jet print head units.
Considering this, the driving voltages to be applied to the head units should be determined individually so that each head unit can eject ink with an optimum ejection speed.
In order to determine the driving voltage, it is conceivable to introduce ink in each head unit. How the head unit ejects ink is monitored while changing the driving voltage applied to the head unit. Printed samples are produced based on the ejected ink, and an observer or worker observes the printed samples. Based on the monitored results and on the observation results of the printed samples, the optimum driving voltage is determined for each head unit.
The above-described conceivable driving-voltage determining method, however, suffers from the problem that the inside of the head unit is stained or smeared with ink when ink is introduced into the print head.
Additionally, the above-described driving-voltage determining method depends on the observer""s ability, and therefore is unreliable.
When a single print head is configured from several head units for several colors of ink, there arises the case where the driving voltages for the respective head units are determined as different from one another. In such a case, the power source provided in the printer has to be configured to supply several different voltages simultaneously. The costs of manufacturing the power source increases.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved method of determining the value of the driving voltage for an ink jet print head unit to adjust the ink ejection condition of the head unit, which does not necessitate introducing ink into the print head, which can determine an optimum driving voltage for a head unit with a simple configuration, and which can set the same, single driving voltage to all the head units provided in a single print head.
In order to attain the above and other objects, the present invention provides a method of determining the value of a driving voltage to be applied to an ink jet print head unit, the ink jet print head unit including a cavity plate and a plurality of piezoelectric elements, the cavity plate being formed with a plurality of pressure chambers and a plurality of nozzles, each pressure chamber being filled with ink and being in fluid communication with a corresponding nozzle, the plurality of piezoelectric elements being provided in one to one correspondence with the plurality of pressure chambers, each piezoelectric element being driven by a driving voltage so as to change the pressure inside the corresponding pressure chamber, thereby allowing ink to be ejected through the corresponding nozzle from the corresponding pressure chamber, the method comprising the steps of: determining at least one of a nozzle-diameter average and a capacitance average, the nozzle-diameter average indicating average of diameters of the plurality of nozzles, the capacitance average indicating average of capacitances of the plurality of piezoelectric elements; and determining a driving voltage to be applied to the piezoelectric elements of the ink jet print head unit by using the determined at least one of the nozzle-diameter average and the capacitance average, and based on a predetermined formula, which is indicative of a relationship of the driving voltage with respect to the at least one of the nozzle-diameter average and the capacitance average.
According to the present invention, therefore, it is possible to easily determine a driving voltage, which should be applied to the ink jet head unit, without actually introducing ink to the ink jet head unit to cause the ink jet head unit to eject ink.
The average-determining step may preferably include at least one of the steps of: determining the nozzle-diameter average, by measuring the diameters of all the nozzles in the ink jet print head unit, and by calculating the nozzle-diameter average based on the measured diameters; and determining the capacitance average, by measuring capacitances of all the piezoelectric elements in the ink jet print head unit, and by calculating the capacitance average based on the measured capacitances. In this case, the driving-voltage determining step calculates the predetermined formula by using the determined at least one of the nozzle-diameter average and the capacitance average, thereby determining the driving voltage.
For example, the nozzle-diameter average can be determined as an arithmetic mean, that is, a quotient obtained by dividing the sum total of the diameters measured for all the nozzles by the number of the nozzles. Similarly, the capacitance average can be determined as an arithmetic mean, that is, a quotient obtained by dividing the sum total of the capacitances measured for all the piezoelectric elements by the number of the piezoelectric elements.
The method may further comprise the step of preparing a plurality of ink jet head units. In this case, the average determining step determines at least one of the nozzle-diameter average and the capacitance average for each ink jet head unit, and the driving-voltage determining step determines the driving voltage to be applied to the piezoelectric elements in each ink jet head unit based on the at least one of the nozzle-diameter average and the capacitance average that is determined for the each ink jet head unit. The method may further comprise the steps of: selecting, among the plurality of ink jet head units, several ink jet head units, for which the driving-voltage determining step has determined the driving voltage of substantially the same values; and assembling together the selected several ink jet head units into a single ink jet print head.
In this way, after preparing a plurality of ink jet head units, an optimum driving voltage is determined for each ink jet head unit in the manner described above. Then, several ink jet head units, for which substantially the same driving voltages have been determined, are selected and assembled together into a single print head. Accordingly, it is possible to apply substantially the same driving voltages to all the head units in the single print head. When manufacturing a printer provided with this print head, it is possible to mount the printer with a power source that supplies a single voltage, only. The costs required for producing the power source can be reduced.
For example, the predetermined formula is indicative of a relationship of the driving voltage with respect to both of the nozzle-diameter average and the capacitance average. In this case, the average-determining step includes both of the steps of determining the nozzle-diameter average; and determining the capacitance average. The driving-voltage determining step determines the driving voltage by using the determined nozzle-diameter average and the determined capacitance average. That is, the driving-voltage determining step calculates the predetermined formula by using the determined nozzle-diameter average and the determined capacitance average.
In this case, the predetermined formula preferably represents a relationship among the driving voltage, the nozzle-diameter average, the capacitance average, a slope of a linear or straight line approximately representing the relationship between the nozzle-diameter average and an ejection-speed average, which is indicative of an average of ejection speeds, at which the plurality of nozzles eject ink, and a slope of another linear or straight line approximately representing the relationship between the capacitance average and the ejection-speed average. In this case, the driving-voltage determining step determines the driving voltage by calculating the predetermined formula based on the calculated nozzle-diameter average and the calculated capacitance average.
It is noted that the slope of the linear line approximately representing the relationship between the nozzle-diameter average and the ejection-speed average is determined previously. Similarly, the slope of the linear line approximately representing the relationship between the capacitance average and the ejection-speed average is also determined previously. The formula indicative of the driving voltage is determined by using these slopes and by using variables representing the nozzle-diameter average and the capacitance average. After calculating the nozzle-diameter average and the capacitance average for the subject ink jet print head, it is possible to determine the driving voltage by substituting those calculated values for the variables in the formula.
The predetermined formula may preferably be represented by E=E0xe2x88x92{xcex1(Cxe2x88x92C0)+xcex2(Dxe2x88x92D0)}/xcex5, wherein xcex1 is the slope of the linear line approximately representing the relationship between the capacitance average and the ejection-speed average, xcex2 is the slope of the linear line approximately representing the relationship between the nozzle-diameter average and the ejection-speed average, D is a nozzle-diameter average variable, C is a capacitance-average variable, D0 is a predetermined nozzle diameter design value, C0 is a predetermined capacitance design value, E0 is a predetermined driving voltage design value, xcex5 is a sensitivity of the ejection speed relative to the driving voltage, and E is the driving voltage. The driving-voltage determining step determines the driving voltage E by substituting the calculated nozzle-diameter average and the calculated capacitance average for the nozzle-diameter average variable D and the capacitance-average variable C, respectively, in the formula.
It is noted that if the ink jet print head unit is produced accurately based on the design values C0 and D0, that is, if each piezoelectric element has a capacitance C0 and each nozzle has a nozzle diameter D0, when the piezoelectric element is driven by the driving voltage design value E0, the piezoelectric element will perform ink-ejection operation through the corresponding nozzle at a desired, ejection-speed design value V0.
On the other hand, if the ink-jet print head unit is produced with their piezoelectric elements having capacitances average C and with their nozzles having nozzle-diameter average D, when the ink-jet print head unit is driven by the driving voltage E that satisfies the above-described formula, the ink-jet print head unit will perform ink-ejection operation at its ejection-speed average being substantially equal to the desired, ejection-speed design value V0. Accordingly, it is possible to easily determine the driving voltage E optimum for the head unit, by merely substituting the calculated nozzle-diameter average for the variable D and by substituting the calculated capacitance average for the variable C.
It is noted that the capacitance average determining step may set the capacitance design value C0 as the capacitance average C. In this case, the driving voltage determining step determines the driving voltage based on the predetermined formula and based on the nozzle-diameter average, the predetermined formula being modified as E=E0xe2x88x92{xcex2(Dxe2x88x92D0)}/xcex5. More specifically, if the ink-jet print head unit is produced with their piezoelectric elements having capacitance average substantially equal to the capacitance design value C0 and with their nozzles having nozzle-diameter average D, when the head unit is driven by the driving voltage E that satisfies the above-described formula, the head unit will perform ink-ejection operation at an ejection-speed average substantially equal to the desired, ejection-speed design value V0. Accordingly, it is possible to easily determine the driving voltage E optimum for the head unit, by merely substituting the calculated nozzle-diameter average for the variable D.
The nozzle-diameter average calculating step may set a nozzle-diameter design value D0 as the nozzle-diameter average D. In this case, the driving voltage determining step determines the driving voltage based on the predetermined formula and based on the capacitance average, the predetermined formula being modified as E=E0xe2x88x92{xcex1(Cxe2x88x92C0)}/xcex5. More specifically, if the ink-jet print head unit is produced with their piezoelectric elements having capacitance average C and with their nozzles having nozzle-diameter average substantially equal to the design value D0, when the head unit is driven by the driving voltage E that satisfies the above-described formula, the head unit will perform ink-ejection operation at an ejection-speed average substantially equal to the desired, ejection-speed design value V0. Accordingly, it is possible to easily determine the driving voltage E optimum for the subject head unit, by merely substituting the calculated capacitance average for the variable C.
According to another aspect, the present invention provides a method of adjusting an ink jet print head unit, the ink jet print head unit being connected to a driving device and including a cavity plate and a plurality of piezoelectric elements, the cavity plate being formed with a plurality of pressure chambers and a plurality of nozzles, each pressure chamber being filled with ink and being in fluid communication with a corresponding nozzle, the plurality of piezoelectric elements being provided in one to one correspondence with the plurality of pressure chambers, each piezoelectric element being driven by a driving voltage applied from the driving device so as to change the pressure inside the corresponding pressure chamber, thereby allowing ink to be ejected through the corresponding nozzle from the corresponding pressure chamber, the method comprising the steps of: calculating a nozzle-diameter average indicating an average of diameters of the plurality of nozzles; calculating a capacitance average indicating an average of capacitances of the plurality of piezoelectric elements; and determining a driving voltage to be applied to the piezoelectric elements based on a predetermined formula that indicates a relationship between the driving voltage and the nozzle-diameter average and the capacitance average.